The invention relates to the field of etch-stop material systems on monocrystalline silicon.
Microelectromechanical systems (MEMS) form the bridge between conventional microelectronics and the physical world. They serve the entire spectrum of possible applications. MEMS include such varied devices as sensors, actuators, chemical reactors, drug delivery systems, turbines, and display technologies. At the heart of any MEMS is a physical structure (a membrane, cantilever beam, bridge, arm, channel, or grating) that is xe2x80x9cmicromachinedxe2x80x9d from silicon or some other electronic material. Since MEMS are of about the same size scale and, ideally, fully integrated with associated microelectronics, naturally they should capitalize on the same materials, processes, equipment, and technologies as those of the microelectronics industry. Because the process technology for silicon is already extensively developed for VLSI electronics, silicon is the dominant material for micromachining. Silicon is also mechanically superior to compound semiconductor materials and, by far, no other electronic material has been as thoroughly studied.
A wide array of micromachined silicon devices are fabricated using a high boron concentration xe2x80x9cetch-stopxe2x80x9d layer in combination with anisotropic wet etchants such as ethylenediamine and pyrocatechol aqueous solution (EDP), potassium hydroxide aqueous solution (KOH), or hydrazine (N2H2). Etch selectivity is defined as the preferential etching of one material faster than another and quantified as the ratio of the faster rate to the slower rate. Selectivity is realized for boron levels above 1019 cmxe2x88x923, and improves as boron content increases.
It should be noted that etch stops are also used in bond and etch-back silicon on insulator (BESOI) processing for SOI microelectronics. The etch-stop requirements differ somewhat from those of micromachining, e.g., physical dimensions and defects, but the fundamentals are the same. Hence, learning and development in one area of application can and should be leveraged in the other. In particular, advances in relaxed SiGe alloys as substrates for high speed electronics suggests that a bond-and-etch scheme for creating SiGe-on-insulator would be a desirable process for creating high speed and wireless communications systems.
Accordingly, the invention provides a SiGe monocrystalline etch-stop material system on a monocrystalline silicon substrate. The etch-stop material system can vary in exact composition, but is a doped or undoped Si1xe2x88x92xGex alloy with x generally between 0.2 and 0.5. Across its thickness, the etch-stop material itself is uniform in composition. The etch stop is used for micromachining by aqueous anisotropic etchants of silicon such as potassium hydroxide, sodium hydroxide, lithium hydroxide, ethylenediamine/pyrocatechol/pyrazine (EDP), TMAH, and hydrazine. For example, a cantilever can be made of this etch-stop material system, then released from its substrate and surrounding material, i.e., xe2x80x9cmicromachinedxe2x80x9d, by exposure to one of these etchants. These solutions generally etch any silicon containing less than 7xc3x971019 cmxe2x88x923 of boron or undoped Si1xe2x88x92xGex alloys with x less than approximately 18.
Alloying silicon with moderate concentrations of germanium leads to excellent etch selectivities, i.e., differences in etch rate versus pure undoped silicon. This is attributed to the change in energy band structure by the addition of germanium. Furthermore, the nondegenerate doping in the Si1xe2x88x92xGex alloy should not affect the etch-stop behavior.
The etch-stop of the invention includes the use of a graded-composition buffer between the silicon substrate and the SiGe etch-stop material. Nominally, the buffer has a linearly-changing composition with respect to thickness, from pure silicon at the substrate/buffer interface to a composition of germanium, and dopant if also present, at the buffer/etch-stop interface which can still be etched at an appreciable rate. Here, there is a strategic jump in germanium and concentration from the buffer side of the interface to the etch-stop material, such that the etch-stop layer is considerably more resistant to the etchant.
In accordance with one embodiment of the invention, there is provided a monocrystalline etch-stop layer system for use on a monocrystalline silicon substrate, the system comprising a graded layer of Si1xe2x88x92xGex and a uniform etch-stop layer of Siyxe2x88x921Gey. In a particular embodiment, the buffer layer is graded up to approximately Si0.8Ge0.2 and a uniform etch-stop layer of approximately Si0.7Ge0.3.
In another embodiment of the invention, there is provided a method of fabricating a monocrystalline etch-stop layer on a silicon substrate comprising depositing a graded buffer layer of Si1xe2x88x92xGex on the silicon substrate; and depositing a uniform etch-stop layer of Si1xe2x88x92yGy on the graded buffer layer. In a particular embodiment, the buffer layer is graded up to approximately Si0.8Ge0.2 on the silicon substrate; and the uniform etch-stop layer of Si0.7Ge0.3 is deposited on the graded buffer layer.
In yet another embodiment of the invention, there is provided a method of micromachining an integrated device comprising providing a silicon substrate; depositing a graded buffer layer of Si1xe2x88x92xGex on the silicon substrate; depositing a uniform etch-stop layer of Si1xe2x88x92yGey on the graded buffer layer; etching portions of the silicon substrate and the graded buffer layer in order to release the etch-stop layer; and processing the released etch-stop layer.